1.You are not a superman.
2.If it's stupid but works, it isn't stupid.
3.Don't look conspicuous - it draws fire.
4.When in doubt, empty your magazine.
5.Never share a foxhole with anyone braver than you are.
6.Never forget that your weapon was made by the lowest bidder.
7.If your attack is going really well, it's an ambush.
8.No plan survives the first contact intact.
9.All 5 second grenade fuses burn down in 3 seconds.
10.Try to look unimportant because the bad guys may be low on ammo.
11.If you are forward of your position, the artillery will fall short.
12.The enemy diversion you are ignoring is the main attack.
13.The important things are always simple.
14.The simple things are always hard.
15.The easy way is always mined.
16.If you are short of everything except enemy, you are in combat.
17.When you have secured an area, don't forget to tell the enemy.
18.Incoming fire has right of way.
Friendly fire isn't.
19.If the enemy is in range - SO ARE YOU!
20.No combat ready unit has ever passed inspection.
21.Things that must be together to work, usually can't be shipped together.
22.Radios will fail as soon as you need fire support desperately.
23.Anything you do can get you shot - including doing nothing.
24.Tracers work both ways.
25.The only thing more accurate than incoming enemy fire is incoming friendly fire.
26.Make it tough for the enemy to get in and you can't get out.
27.If you take more than your fair share of objectives, you will have more than your fair share of objectives to take.
28.When both sides are convinced that they are about to lose, they are both right.
29.Professional soldiers are predictable, but the world is full of amateurs.
30.Murphy was a grunt.
Your weapon is your best friend, it is your only friend, and to be more effective soldier, you must master your weapon! -Drill Sergeant T.Barnes.

I would bother explaining a few un-confidential SCPs. But everyone knows who SCP 173, 096, 049, 683, etc, so I'm just leaving my profile as an obvious sign I'm a fan of SCP.